use version number instead of release in .travis.yml

[tkelman]

release will change over time, but your REQUIRE file says this package supports julia 0.5 so it should continue to be tested

[coveralls]

Coverage remained the same at 57.285% when pulling 41faad7 on tkelman:travisver into 3a10eb7 on JuliaOpt:master.

[codecov-io]

Codecov Report

Merging #18 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #18   +/-   ##
=======================================
  Coverage   57.28%   57.28%           
=======================================
  Files           6        6           
  Lines         302      302           
=======================================
  Hits          173      173           
  Misses        129      129

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