BUILD: travis-ci: trigger non-mainstream configurations only on daily builds.

BUILD: travis-ci: trigger non-mainstream configurations only on daily builds.

Let us save some electricity of Travis-CI.

The following configurations are built on every push:

  - linux-glibc with threads+openssl 1.1.1 on x86 + clang
  - linux-glibc with threads+libressl 2.9.2 on x86 + clang
  - linux-glibc without SSL nor threads on x86 + clang
  - osx with openssl-1.1.1 + clang

The following configurations are built daily:

  - linux-ppc64le + openssl-1.0.2
  - linux + openssl-1.1.0 + 51d/tree
  - linux + libressl-2.8.3
  - linux + libressl-2.7.5 + prometheus
  - linux + boringssl
  - cygwin
  - coverity scan