Install lzma compression library for Ag

Installs liblzma-dev which is required for *The Silver Searcher* on Ubuntu 13.04 Raring.  Without this, installation fails with the following error:

```
checking for LZMA... no
configure: error: Package requirements (liblzma) were not met:

No package 'liblzma' found

Consider adjusting the PKG_CONFIG_PATH environment variable if you
installed software in a non-standard prefix.

Alternatively, you may set the environment variables LZMA_CFLAGS
and LZMA_LIBS to avoid the need to call pkg-config.
See the pkg-config man page for more details.
failed
make: *** No rule to make target `install'.  Stop.
failed
```