Remove Jeweler and clean up gemspec

[mishina2228]

The Jeweler task was removed many years ago. 4b7e14b
But only the dependency remained.
This PR removes Jeweler and cleans up gemspec.
Here are the details:

• date, rubygems_version and specification_version should not be set
  • RubyGems say so. date, rubygems_version, specification_version
• Remove redundant required_rubygems_version
• Remove deprecated test_files and exclude test files from files
  • Please see the following for details.
    • Documentation about 'test_files' is confusing rubygems/guides#90
    • Don't package test files with gem rubygems/bundler#3207

[joshuacronemeyer]

We still use jeweler. It gives us some handy rake tasks by default like rake install and rake release and I use those when i make a new release. However I am interested in the gem spec cleanup you did. Can u update the PR to just cleanup gem spec?

[mishina2228]

We still use jeweler. It gives us some handy rake tasks by default like rake install and rake release and I use those when i make a new release.

No worry, both rake install and rake release are given from Bundler.

$ bundle exec rake -T
rake build            # Build timecop-0.9.5.gem into the pkg directory
rake build:checksum   # Generate SHA512 checksum if timecop-0.9.5.gem into the checksums directory
rake clean            # Remove any temporary products
rake clobber          # Remove any generated files
rake clobber_rdoc     # Remove RDoc HTML files
rake default          # Default: run tests
rake install          # Build and install timecop-0.9.5.gem into system gems
rake install:local    # Build and install timecop-0.9.5.gem into system gems without network access
rake rdoc             # Build RDoc HTML files
rake release[remote]  # Create tag v0.9.5 and build and push timecop-0.9.5.gem to rubygems.org
rake rerdoc           # Rebuild RDoc HTML files

It may not be exactly the same as what Jeweler provides, but it probably won't be a problem.

[mishina2228]

Scheduled CI fails now, but the reason is as noted at #397 (comment).
CI should be green if both this PR and #397 are merged.