Fixed performance issue by avoiding to call s:py_core_load() if disabled.

Before the change the startup time looked like this:
vim --startuptime vim.log -c:q && vim vim.log
111.519  003.536  002.629: sourcing ~/.vim/bundle/vim-easymotion/plugin/EasyMotion.vim
112.412  000.592  000.592: sourcing ~/.vim/bundle/trans.vim/autoload/trans.vim
152.550  000.358  000.358: sourcing ~/.vim/bundle/trans.vim/autoload/trans/data.vim
152.781  041.140  040.190: sourcing ~/.vim/bundle/trans.vim/plugin/trans.vim
153.267  000.187  000.187: sourcing /usr/share/vim/vim73/plugin/getscriptPlugin.vim

And now it looks like this if you set g:trans_has_python = 0 before the plugin
is loaded:
109.487  003.587  002.720: sourcing ~/.vim/bundle/vim-easymotion/plugin/EasyMotion.vim
110.373  000.585  000.585: sourcing ~/.vim/bundle/trans.vim/autoload/trans.vim
111.051  000.320  000.320: sourcing ~/.vim/bundle/trans.vim/autoload/trans/data.vim
111.216  001.608  000.703: sourcing ~/.vim/bundle/trans.vim/plugin/trans.vim
111.589  000.095  000.095: sourcing /usr/share/vim/vim73/plugin/getscriptPlugin.vim

vim --cmd 'profile start trans.profile' --cmd 'profile! file ~/.vim/bundle/trans.vim/*' -c 'q' && vim trans.profile
    1              0.000006     if !exists("g:trans_has_python") || g:trans_has_python != 0
    1              0.000004     if has("python") "{{{
    1   0.000026   0.000008         call trans#default("g:trans_has_python", 2)
    1              0.000003         let s:py="py"
    1   0.038763   0.000012         call s:py_core_load()
    1              0.000005     elseif has("python3")
                                    call trans#default("g:trans_has_python", 3)
                                    let s:py="py3"
                                    call s:py_core_load()
                                else
                                    let g:trans_has_python = 0
                                endif "}}}
    1              0.000001     endi