Improvements to Installation documentation page

[ifdattic]

In my opinion the installation page is a bit outdated and could be improved/simplified. Here are the changes I would suggest:

• Remove the note about requiring PHP 5.3+. Or change because as far as I can tell the documentation is for the latest version and latest version requires at least 5.6
• Remove Method Convert to PHP 5.3 #1 as only a single method is available
• Replace composer install with simple "Run the composer require command ....."
• Because examples in documentation use the bin/phpspec command add the section which tells how to change the bin directory and the documentation uses this directory. That by default the command would be at vendor/bin/phpspec
• Instructions for installing composer itself (the link to its installation page) could be left as is.

[ciaranmcnulty]

I agree about PHP versions.
I agree about the #1.

I'm unsure about changing all the examples to vendor/bin/phpspec as I always set the bin-dir, but we could indeed update the install instructions to use composer commands rather than the JSON

[ifdattic]

Might have not translated in my comment, but personally myself I think it's not worth changing all examples to vendor/bin/phpspec. That I'm suggesting is just a adding a section explaining that it's possible to change the directory of the executable. If we want to get even fancier we can add that you can make a bash alias to just execute with phpspec :)

[Sam-Burns]

Fixed in #919