Opening files doesn't work on mac

[mgtlake]

Seems to be an issue if .jl extension doesn't have an associated editor - probably can be resolved by choosing a default editor if open fails.

No application knows how to open /Users/user/Juliet/Functions/1.jl.
ERROR: failed process: Process(`open /Users/user/Juliet/Functions/1.jl`, ProcessExited(1)) [1]
 in run at /Applications/Julia-0.4.6.app/Contents/Resources/julia/lib/julia/sys.dylib
 in setup_function_file at /Users/user/.julia/v0.4/Juliet/src/Juliet.jl:301
 in complete_lesson at /Users/user/.julia/v0.4/Juliet/src/Juliet.jl:196
 in choose_lesson at /Users/user/.julia/v0.4/Juliet/src/Juliet.jl:121
 in juliet at /Users/user/.julia/v0.4/Juliet/src/Juliet.jl:74