Slepc 3.5.2 (update)

Update SLEPc to the newest version.
Add optional dependency on Arpack.
here is what i have (i installed the first complex version before modifying the script):
```
brew info slepc
slepc: stable 3.4.4
http://www.grycap.upv.es/slepc
/usr/local/Cellar/slepc/3.5.2 (928 files, 7.9M)
  Built from source with: --with-arpack
/usr/local/Cellar/slepc/3.5.2-real (928 files, 7.9M) *
  Built from source with: --with-arpack
```