maint - repopulate the travis matrix of tested versions

7ca85a8 dropped travis testing of 3.0, 3.1, and 3.2, and also ruby 1.8.7.

7ff5aae claims that 3.0, 3.1, and 3.2 are tested and working versions.

This commit reinstates these jobs to the travis matrix so we can have some
more confidence that the claims of 7ff5aae are substantiated.