fix(select): Fix SassC compilation error

[vzctl]

SassC::SyntaxError compilation error is caused by unquoted backround-image url:

SassC::SyntaxError: Error: expected a variable name (e.g. $x) or ')' for the parameter list for url
        on line 17987 of stdin
>>   background-image: url(data:image/svg+xml,%3Csvg%20width%3D%2210px%22%20heigh
   -----------------------^
stdin:17987
/home/lex/.gem/ruby/2.3.0/gems/sassc-1.11.4/lib/sassc/engine.rb:47:in `render'
/home/lex/.gem/ruby/2.3.0/gems/sassc-rails-1.3.0/lib/sassc/rails/compressor.rb:16:in `call'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/sass_compressor.rb:28:in `call'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/processor_utils.rb:75:in `call_processor'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/processor_utils.rb:57:in `block in call_processors'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/processor_utils.rb:56:in `reverse_each'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/processor_utils.rb:56:in `call_processors'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/loader.rb:134:in `load_from_unloaded'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/loader.rb:60:in `block in load'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/loader.rb:317:in `fetch_asset_from_dependency_cache'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/loader.rb:44:in `load'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/cached_environment.rb:20:in `block in initialize'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/cached_environment.rb:47:in `load'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/base.rb:66:in `find_asset'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/base.rb:73:in `find_all_linked_assets'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/manifest.rb:142:in `block in find'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/legacy.rb:114:in `block (2 levels) in logical_paths'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/path_utils.rb:228:in `block in stat_tree'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/path_utils.rb:212:in `block in stat_directory'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/path_utils.rb:209:in `each'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/path_utils.rb:209:in `stat_directory'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/path_utils.rb:227:in `stat_tree'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/legacy.rb:105:in `each'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/legacy.rb:105:in `block in logical_paths'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/legacy.rb:104:in `each'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/legacy.rb:104:in `logical_paths'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/manifest.rb:140:in `find'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/sprockets/manifest.rb:185:in `compile'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-rails-3.2.1/lib/sprockets/rails/task.rb:68:in `block (3 levels) in define'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-3.7.1/lib/rake/sprocketstask.rb:147:in `with_logger'
/home/lex/.gem/ruby/2.3.0/gems/sprockets-rails-3.2.1/lib/sprockets/rails/task.rb:67:in `block (2 levels) in define'
/home/lex/.gem/ruby/2.3.0/gems/rake-12.3.0/exe/rake:27:in `<top (required)>'
Tasks: TOP => assets:precompile

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[vzctl]

I signed it!

[googlebot]

CLAs look good, thanks!

[kfranqueiro]

Thanks for the PR! I took a look at this and it works as intended with no unintended side effects.

The Travis CI failure only concerns unit tests (which this PR doesn't affect) and is due to environment variables not exposed to forks.