Mesa 7.6 is a new development release. People who are concerned with stability and reliability should stick with a previous release or wait for Mesa 7.6.1.
Mesa 7.6 implements the OpenGL 2.1 API, but the version reported by glGetString(GL_VERSION) depends on the particular driver being used. Some drivers don't support all the features required in OpenGL 2.1.
See the Compiling/Installing page for prerequisites for DRI hardware acceleration.
Note that the Mesa project is no longer using odd/even version numbers to indicate development/stable releases. The so-called development releases have been fairly stable. If you're especially concerned with stability you should probably look for "point" releases such as 7.5.1 which will be a bug-fix release.
tbd