desugar-check follows its postcondition now

Check blocks, if nested, would remain in the AST, causing Pyret
to violently throw up when they appeared. This bug is responsible
for test failures on master.
Small modification to travis config

This should prevent build times from hitting the 50-minute
limit (or even coming close.)
Merge pull request #232 from brownplt/faster-travis

Small modification to travis config
Fix parsing and checking empty record annotations.