[master] Fix composer package name should be all lowercase

[jfcherng]

Proposed changes

Composer 1.8.2 (latest) would now emit a deprecation warning that package name should in lowercase.

So I think it would be better to change the install command in the README.

Deprecation warning: require.phpFastCache/phpfastcache is invalid, it should not contain uppercase characters. 
Please use phpfastcache/phpfastcache instead. Make sure you fix this as Composer 2.0 will error.

Types of changes

What types of changes does your code introduce to Phpfastcache?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing code/behavior)
• Deprecated third party dependency update
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation/Typo/Resource update that does not involve any code modification

Agreement

I have read the CONTRIBUTING and CODING GUIDELINE docs

[Geolim4]

Thanks for your contribution :)

[Geolim4]

Also you can make me a PR on v5/v6 if you want :)

[jfcherng]

@Geolim4 Sure. My pleasure.