doc install: add more descriptions to build from source
doc install: add more description about Groonga's development package
doc ja: remove garbage "<"
doc: use only GitHub for accepting problem reports

GitHub issues is easy to submit a report rather than our
Redmine. Because reporters don't need to create a new account for our
Redmine.

TODO:

  * Translate me!
travis: use released versions
travis: restrict the max concurrency
travis: remove needless "$"...
travis: fix wrong true literal
travis: disable allowing failure of building with system MySQL
travis: remove needless concurrency restriction for MariaDB bundled build
travis: install before test when MariaDB bundled build
travis: use the same test usage for MariaDB bundled build
travis: fix cmake option
travis: debug
travis: use MROONGA_ prefix to avoid variable name conflict
travis: disable concurrency build for MariaDB bundled build

    make[3]: warning: jobserver unavailable: using -j1.  Add `+' to parent make rule.
travis: disable debug print
travis: enable embedded test
travis: add -DWITH_EMBEDDED_SERVER=TRUE for embedded server test build
travis: disable only parallel build