install development requirements for wal-e deps

In https://github.com/wal-e/wal-e/commit/ebd482a85418601372f31648c4b6473e5eb1db06 (version 0.7.0 and higher) new Python package
dependencies were included.
These deps depend on some further development libraries for successful
compilation.

Namely: libxslt1-dev and libxml2-dev - both are provided by a virtual
package libxslt-dev. This package exists on 10.04, 12.04 and 14.04 as a
helper for the namespace to provide the correct packages.