travis-ci: Run make distcheck during travis testing

Use 'make distcheck' instead of 'make && make check' during travis-ci
builds. This adds an extra layer of checks to the CI testing, including
ensuring everying is in 'make dist' that is required to build and
test the code, and also that a VPATH build and test still works.
Add tap-driver.sh to list of EXTA_DIST files
build: do not install build-generated curve keys

Keys generated during build should probably not be installed
during make install. This also fixes `make install` error from
VPATH build due to listing a directory instead of files in
sysconf_DATA.

Fixes #53
Merge pull request #54 from grondo/distcheck-fixups

fixes for make distcheck