installation.rst: Update a console script path and remove doctrine:phpcr:fixtures:load

[Yrwein]

Just two small fixes:

• the location for a console script is app/console, not bin/console.
• doctrine:phpcr:fixtures:load command is no longer needed (and it kicks an error) according to Could not find any fixtures to load in: standard-edition#59

[dbu]

thanks for helping out. we meant to replace that documentation but the change was not merged yet. it is now:

we moved the book into the main documentation in #835 which i just merged. the book as it was written would not work anymore with the standard edition, because of the missing fixtures. we now offer the quick tour as a tutorial-style introduction, using the sandbox.

the console lives in bin in the master version of standard edition: https://github.com/symfony-cmf/standard-edition/tree/master/bin