Suggestion for reducing Travis CI matrix

[chrisr-diffblue]

In an email, @andreast271 suggested a possible reduced build matrix for travis that would still give us roughly the same coverage:

The current build configurations are:

OS	Compiler	Build System	DEBUG defs	Regression
linux	gcc	make		yes
osx	gcc	make		yes
osx	clang	make		yes
linux	gcc	make	DEBUG	no
linux	clang	make		yes
linux	clang	make	DEBUG, no USE_DSTRING	no
linux	gcc	cmake		yes
osx	gcc	cmake		yes

Andrea's suggested new build matrix:

OS	Compiler	Build System	DEBUG defs	Regression
linux	gcc	make		yes
osx	gcc	make		yes
linux	clang	make		yes
linux	clang	make	DEBUG, no USE_DSTRING	no
linux	gcc	cmake	NDEBUG	yes
osx	clang	cmake		yes

[chrisr-diffblue]

No longer using travis for CI (several generations old!)