Configure continous integration (Travis).

Changes permission after making sure the composer binary is in place
(downloaded or self-updated).
For database environments, only use default database password if
environment variable is not set.

Travis (Continous Integration)
------------------------------

- Badge on README now uses SVG instead of PNG.
- Installs and configure Apache+PHP-FPM.
- Removes PHP 5.3 from build matrix, since it is obsolete.
- Adds PHP 5.5 to the build matrix.
- Adds two different environments for each PHP version:
  - MySQL database.
  - SQLite database (this sucker won't work, see GH#2).