fix gemspec to allow Rails5

[akito19]

Hi!

When I upgraded to Rails5.0 in my project, exception_notification said

$ bundle update rails
Fetching gem metadata from https://rubygems.org/
Fetching version metadata from https://rubygems.org/
Fetching dependency metadata from https://rubygems.org/
Resolving dependencies.........
Bundler could not find compatible versions for gem "actionmailer":
  In Gemfile:
    exception_notification was resolved to 4.1.4, which depends on
      actionmailer (~> 4.0)

So I rewrote exception_notification.gemspec and added /gemfiles/rails5_0.gemfile.
Thank you!

[coveralls]

Coverage remained the same at 94.216% when pulling 01fe8d7 on akito19:fix_gemspec_to_upgrade_rails_version into 4d6adef on smartinez87:master.

[coveralls]

Coverage decreased (-0.7%) to 93.563% when pulling ae1f86e on akito19:fix_gemspec_to_upgrade_rails_version into 4d6adef on smartinez87:master.

[coveralls]

Coverage decreased (-0.7%) to 93.47% when pulling 2c2e1a4 on akito19:fix_gemspec_to_upgrade_rails_version into 4d6adef on smartinez87:master.

[coveralls]

Coverage decreased (-0.7%) to 93.47% when pulling 2c2e1a4 on akito19:fix_gemspec_to_upgrade_rails_version into 4d6adef on smartinez87:master.

[smartinez87]

thanks @akito19 for your work! We just merged #306, so I'll close this one