Support Middleman v4.4.x

[ProgrammingIncluded]

Reproduction

Cannot install library on Middleman 4.4.x due to activerecord version mismatch:

middleman-blog-similar (~> 2.0.1) x64-mingw32 was resolved to 2.0.1, which
depends on
activerecord (~> 5.0, >= 5.0.0) x64-mingw32 was resolved to 5.0.1.rc1,
which depends on
        activesupport (= 5.0.1.rc1) x64-mingw32

    middleman (~> 4.4.2) x64-mingw32 was resolved to 4.4.2, which depends on
middleman-core (= 4.4.2) x64-mingw32 was resolved to 4.4.2, which depends
on
        activesupport (>= 6.1, < 7.0) x64-mingw32

Fix

• Bump activerecord to >= 6.1 which auto updates activesupport >= 6.1
• Modify article.rb to use new API: to_hash -> to_ary
• Update middleman-blog-similar VERSION to 2.1.0 due to dependency upgrade.

Possible Issues

• Not sure if middleman version 4.4.x is the minimum version for activesupport >= 6.1.
• Have not tested initialization code for possible API breakage due to existing SQL database.

This PR was created with minimal testing. Please use as much or as little as possible for the changes. I have found your library to be very helpful so would appreciate it being updated to support latest middleman.

Best,
PI