install_prereq: Checkout of libSRTP 1.5.x.

Since 5th November 2014, the master branch of libSRTP changed the prefix of
several member names and is not compatible with the source code in Asterisk
anymore. Therefore instead, this change checks out the latest version of the
libSRTP 1.5.x branch. Furthermore now, libSRTP is compiled with OpenSSL as
backend. This makes AES-GCM and AES-IN possible.

ASTERISK-22131 #close

Change-Id: I2e396cdc01da0ff610686e398ed210ca7408f7d6