check whether blast is in the path on load

Fixed #50 

Checks if BLAST is in the path as soon as genevalidator starts... (i.e. before any output files are created).

In the Travis CI config, I have added a few lines so that a blank blastp file is added to the path and as a result, the tests are able to be run on Travis without BLAST being installed...

NOTE: This logic assumes that `blastp` and `blastx` must both be installed together (it runs `which` on just `blastp`).  I cannot think of a case where you would only install one of two executables...