autotools: quote $(RELEASE) to avoid syntax errors

Otherwise you may see errors like this:

/bin/bash: -c: line 0: syntax error near unexpected token `('
/bin/bash: -c: line 0: `echo "  GEN   " munin-node-c.1;pod2man --section=1 --release=0.0.6-(detached-2013-10-24-c33-g3889db3 --center="Munin C Documentation" munin-node-c.pod > munin-node-c.1'
getversion: fix handling of detached commits

The inline documentation of getversion claims to use git describe for
detached heads. Presumably git's output for detached heads changed from
something that starts with "(no" to something that starts with
"(detached" now. Contrary to the documentation, detached branches were
handled like feature branches with a "(" in their name which produces a
version string such as:

0.0.6-(detached-2013-10-24-c12-gabcdef

The opening brace breaks autoreconf like this:

/usr/bin/m4:configure.ac:11: ERROR: end of file in argument list
travis: skip "make test"

The implicit default value for script is

./configure && make && make test

but we don't currently have a test target, so travis builds fail.
travis: also build using clang

Note that building with clang currently fails, because we have warnings
about unused arguments to $CC and -Werror.
Merge branch 'travis'

This branch adds configuration for the travis-ci.org continuous
integration service and fixes two aspects of the build systems
identified by travis.