Fix failure message in tests

This got broken by Emoji::Character API change in
3935375ca8472cb34b6cbab92baa0180167ce6f9
Improve failure message for images on disk test
Rename PNGs on disk to not contain VARIATION_SELECTOR_16

Even if the emoji contains it, the filename on disk doesn't have to.
This will reduce having to keep renaming PNGs on disk while we tweak the
emoji database in the future in regard to which are required to have
VARIATION_SELECTOR_16 and which are not.
Don't treat plain "(c)" symbol or similar as emoji

Unless these characters are followed by VARIATION SELECTOR-16, which
indicates that the should be rendered as emoji, OS X renders these
characters using plain font glyphs.

Previously we assumed that the emoji list we get from
`Category-Emoji.json` (OS X emoji character picker) are all valid emoji,
but for some reason some plain Unicode characters are included in there
without being suffixed with VARIATION SELECTOR-16.
Ruby 1.9 compat in tests
Merge pull request #58 from github/plain-copyright

Don't treat plain "(c)" symbol or similar as emoji