Update README.md

The require 'activerecord' was not working for me, on searching I found the solution
here http://stackoverflow.com/questions/8981576/error-loading-active-record-gem-with-sinatra-app-using-rvm
I have proposed the changes for the same.
Support dirty tracking with rails 3.2.9.rc1

Starting with rails 3.2.9.rc1 the method #field_changed?
was renamed to #_field_changed?

https://github.com/rails/rails/pull/3544
Merge pull request #237 from ankitdhingra/patch-1

Update README.md
Clear test database before running rake spec
Fix pending specs on Oracle 10g
Fix pending specs on Oracle 10g properly
Update History.md
Release 1.4.2.rc1
Remove_column should raise an ArgumentError when no columns are passed
Address 2 failues since numeric_changes_from_zero_to_string? method implemented at Rails.
Address test_integer_zero_to_integer_zero_not_marked_as_changed failure

See rails issue #7237.
Merge pull request #246 from yahonda/remove_column_should_have_columns

Remove_column should raise an ArgumentError when no columns are passed
Merge pull request #247 from yahonda/numeric_column_changes_v142

Address numeric column changes failures
Fix rails issue 6819.
Address 2 failures since c4601179541a9fc2e3cd7828335cefe54f26969a merged.
Merge pull request #248 from yahonda/rails6819_v142

Address schema dumper test failure
Update History.md to show 1.4.2 changes
Merge pull request #249 from yahonda/update_readme_v142

Update History.md to show 1.4.2 changes
don't specify column type in schema dump for NUMBER virtual columns
fix typo