Merge #722: Context isn't freed in the ECDH benchmark

85b35afa765ba23ac3682cf15800769b0a3b834d Add running benchmarks regularly and under valgrind in travis (Elichai Turkel)
ca4906b02e69644d83e13b3c09cc9147fc1232a5 Pass num of iters to benchmarks as variable, and define envvar (Elichai Turkel)
02dd5f1bbb3af3660ecff276c3a108371979b67c free the ctx at the end of bench_ecdh (Elichai Turkel)

Pull request description:

ACKs for top commit:
  real-or-random:
    ACK 85b35afa765ba23ac3682cf15800769b0a3b834d I looked at the diff and tested the ecdh benchmark using valgrind
  jonasnick:
    ACK 85b35afa765ba23ac3682cf15800769b0a3b834d

Tree-SHA512: 029456d2c8f6c2c45c689279683ae30b067872bbfaee76a657f7fc3a059e2dffcde09ed29e3610de3adb055405126b6c3656c7ab5f5aaa1f45af2b32d4693ec4