Added consistency for setting install path

- for configure, the install directory is <prefix>/genesis-2.4
- for Makefile.dist, introduced PREFIX variable:
  `PREFIX=<path> make install` installs to <prefix>/genesis-2.4
- in both cases, can override target path by calling
  `INSTALLDIR=<path> make install`

Also, fixed travis-ci rule (don't use relative path for configure!)