Don't depend on Lisp implementation support for ~ in pathnames

At least CMUCL doesn't support it.
No need to apt-get update if we're not adding a dpkg architecture
Add support for ABCL
Simplify CMUCL setup

The ASDF bundled with CMUCL is fine. The issues I was seeing were due to
the cl-asdf package installed in my local Ubuntu system.
Minor cosmetic improvement
Make sure that each $LISP is available in the PATH
Add support for ECL
Register ~/lisp and $TRAVIS_BUILD_DIR in ASDF's source registry
Merge newest from branch 'luis/master' into patch-1

Conflicts:
	.travis.yml