Test against modern rubies.

Test against 2.0.0 and 2.1.1. Scratch 1.8.x. Keep 1.9.3 for people
lagging behind on upgrading, but allow the build to fail on it.
Remove incompatible rcov

rcov is not compatible with ruby 2.x
Bump mini_portile dependency

This allows users to update other gems which depend on new versions of
mini_portile.
Merge pull request #44 from tamird/master

Bump mini_portile dependency
Update libgpg-error and gpgme versions
Update tarballs
Expose new functions / constants in GPGME 1.5.0
Update to 2.0.6
add coveralls
travis: Output compile.log after failure
travis: Use 'find' to collect compile.log
travis: Fix validation error
Disable gpgsm test to work around a build error on Travis CI
Check if the curve field of gpgme_subkey_t is non NULL
Merge pull request #46 from tamird/simplecov

Coveralls
Add Coverall status badge to README
build: Replace -lLIB with LIB.a for locally built libraries

LIBPATH in the generated Makefile may contain -L$(archlibdir) before
the ports' lib directories.  Disable path searching by resolving absolute
paths of locally built libraries.

Fixes issue #50.
Update tarballs
build: Update libassuan and gpgme version
Follow the 1.5.1 interface change