[FIX] libc++ test fixes

`SequestOutfile_test` and `MRMDecoy_test` fail when compiled with libc++ (default on mac os x >= 10.9). This pull request fixes both tests. 
can you document
'pseudoRNG( (pI_it - peptide_index.begin() ) + 1 )'
a little please? My first impression was that it can lead to index-out-of-bounds (which it cannot, luckily).

It takes some time to find that it actually generated RNG's in an half-open interval from 0 to n (i.e, generate(eng, 0, n - 1, n - 1); ), as opposed to just calling pseudoRNG(), which uses an open interval.