Add valgrind run to CI

[afk11]

as mentioned in https://github.com/Bit-Wasp/secp256k1-php/issues/131, we should have travis run valgrind tests.

recently I've done some work on this using a php build from source with the following: ./buildconf && ./configure --with-valgrind --with-pcre-valgrind --with-secp256k1 && make -j8

with the projects secp256k1 dir located in ./ext/secp256k1 and then using the following to run valgrind tests: make test TESTS="./ext/secp256k1/tests -m"

ideally we'd be able to run this on multiple php versions - maybe several Docker files in phpqa?

[afk11]

seems a debug build of php mightn't be necessary (for simply testing for issues anyway), so travis' php should be suitable.