Use flymake-easy for flymake support

Related to purcell/less-css-mode#12, this change prevents crashes on invalid imports. It doesn't actually detect the errors, but it keeps flymake going.

Note that lessc v1.4.0 changes the output format anyway, so a perfect error regex would detect all of the following:

Normal errors in v1.3.3:

    NameError: variable @red is undefined in /home/wilfred/scratch/foo.less:4:11

Import errors in v1.3.3:

    FileError: 'noexist.less' wasn't found.
     in /home/wilfred/scratch/foo.less:1:0

All types of errors in v1.4.0:

    NameError: variable @red is undefined in /home/wilfred/scratch/foo.less on line 4, column 12: