add test demonstrating there is a bug in pgorder calculation

pgorder for the LJ75 global minimum is 20 according to pathsample, but
we compute 60.  This is most likely a bug.
Merge branch 'master' of github.com:pele-python/pele
stop the rate tests from sporadically failing

Add a small lj15 database for use in the rate tests
Install more test modules and data files used by the tests.
Travis-CI test file.