Qt 5.12.1 and valgrind on Linux (#306)

Qt 5.12.1 and valgrind on Linux (#306)

set an environmental variable when running valgrind to disable QRegularExpressions use of JIT which causes problems for valgrind. The release build of Qt 5.12.1 uses JIT by default. The debug build does not.

warn when running older version of valgrind that don't handle RDRAND instructions used by newer versions of Qt such as 5.12.1.

support multiple docker images in travis.

create a travis docker image with Qt 5.12.1.