Support Julia v0.6

[isuruf]

See failed log, http://pkg.julialang.org/logs/SymEngine_0.6.log

[jverzani]

I think most of these are minor compat issues, but the error comes from something that currently isn't working, but should be fixed. It is in how the lambdify function makes functions. They need compiling before usage leaving the form lambdify(ex)(val) throwing that error. (https://discourse.julialang.org/t/help-calling-a-function-defined-from-expressions/1340/5). A simple fix is to assign first: fn = lambdify(ex); fn(val) but until the underlying issue is addressed the plot recipes won't work either.