Update INSTALL.md

- Changing the Composer installation command to install the SWM 1.9.x branch of SMW (using the notation people find on packagist.org)
- Formatting the invocation of SMW in case of manual installation
- Fixing a link to documentation page (Upgrading)
Update INSTALL.md

form to the composer command
Merge pull request #61 from kghbln/patch-2

Update INSTALL.md