Unit tests sometimes generate a time-out error

[unzvfu]

First noticed during assessment of #56.
The tests sometimes pass, sometimes fail. Failure is always due to exceeding the 10m time limit.
Example: https://travis-ci.org/n1analytics/javallier/jobs/257569748
Failures are usually on OpenJDK7, seen once on OracleJDK7, never on OracleJDK8.

One idea: Some tests use the system entropy (cf. #58), which will block when the random material is depleted.

[unzvfu]

An experiment that removes use of SecureRandom seems to fix the problem: https://travis-ci.org/n1analytics/javallier/builds/267016134
Unfortunately, modifying the system so that it is parameterised by PRNG will be a big job (cf. #58 (comment) )

[unzvfu]

This is apparently fixed by #62.