Added 'in' option for integer filter.

Similar to string filter, it matches an array of allowed values.
Add a duck type to handle methods that need to respond to certain methods
Merge branch 'master' of github.com:cypriss/mutations
Merge pull request #17 from tomtaylor/master

Added 'in' option to integer filter.
Write a file filter
Bump version, add a change log
Add 1.8.7 support. Thx to @tfausak
Remove required ruby version from gemspec
Add minitest to the test group in bundler
Use the proper travis filename
Try all the rubies
Update changelog