Fix infinite recursion in conversion

[fingolfin]

Fixes #165
Fixes #167

[fingolfin]

While PR #168 fixes the issues in #167 by adding the requested conversion, this PR fixes the other aspect, namely that an unsupported conversion lead to a crash -- now it leads to an error. E.g. this GAP code now errors, instead of crashing:

GAPToJulia( JuliaEvalString( "String" ), "a" );

And so does this Julia code:

GAP.gap_to_julia( Dict{Int64,Int64}, GAP.julia_to_gap( "a" ) )  // used to crash
GAP.gap_to_julia( Dict{Int64,Int64}, 1 ) // used to produce StackOverflowError

[codecov]

Codecov Report

Merging #169 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #169   +/-   ##
=======================================
  Coverage   54.72%   54.72%           
=======================================
  Files          37       37           
  Lines        1683     1683           
=======================================
  Hits          921      921           
  Misses        762      762

Impacted Files	Coverage Δ
LibGAP.jl/src/gap_to_julia.jl	95.76% <100%> (ø)	⬆️

[sebasguts]

I will merge this once travis is finished