Exclude spec files from gem package

[amatsuda]

Here's a patch that excludes spec files from the gem package.

With this patch, the .gem file size diminishes from 28672 bytes to 23040 bytes on current master.

[imanel]

Interesting, looks like somewhere along the way test_files was dropped from the spec. Good catch.

Test in 2.1 failed because grep_v was added in Ruby 2.3. Could you switch to grep with negative regex or to select / find_all?

[amatsuda]

Test in 2.1 failed because grep_v was added in Ruby 2.3. Could you switch to grep with negative regex or to select / find_all?

Oops! Pushed another commit doing this.

[imanel]

Thanks!