[DOC] install.rst: numexpr and anaconda warnings

- Anaconda currently provides `pandas` v0.13, `numpy` 1.7.1 and `numexpr` 2.2.2 as the latest versions (#6056).
- There seem to exist rare but evil lurking bugs when using `numexpr` <= 2.2.2 and `numpy` >= 1.8.0 (#6119).