travis: use -j 4 to build cppcheck, the testrunner and cppcheck-gui.
make install: install cppcheck-htmlreport as well.
travis: also check "make install"
Merge pull request #179 from matthiaskrgr/travis_speedup

travis: use -j 4 to build cppcheck, the testrunner and cppcheck-gui.
Merge pull request #178 from matthiaskrgr/make_install

Make install: install cppcheck-htmlreport and cppcheck-gui (if binary is compiled), make travis also perform make install.