Update History.txt for 0.5.1
Update History.txt for 0.5.1

JDominator/ruby-plsql,rsim/ruby-plsql,jgebal/ruby-plsql,rsim/ruby-plsql,JDominator/ruby-plsql,jgebal/ruby-plsql,jgebal/ruby-plsql,yahonda/ruby-plsql,yahonda/ruby-plsql