Run CRL spec tests in serial.

Because there is now a ReadWriteLock on the CRL file, it is possible
for one thread to hold the file lock and delay the writing of a serial
to the CRL.  If the spec tests are running in parallel, the delayed
write may cause a failure for one of the tests that checks for a new serial
in the CRL.
Merge pull request #469 from candlepin/awood/crl-in-serial

Run CRL spec tests in serial.