Support Gurobi 5.0 and show an error if no binary found

Corrects a problem I had: set the GUROBI_HOME environment variable, installed the Gurobi package, without problem, but unable to import the module! The error message urged me to install the package, but it happened without error: 
´´´
julia> import Gurobi
ERROR: Gurobi not properly installed. Please run Pkg.build("Gurobi")
 in error at error.jl:21
 in include at ./boot.jl:245
 in include_from_node1 at ./loading.jl:128
 in reload_path at loading.jl:152
 in _require at loading.jl:67
 in require at loading.jl:51
while loading /home/cuvelier/.julia/v0.3/Gurobi/src/Gurobi.jl, in expression starting on line 6
julia> Pkg.build("Gurobi")
INFO: Building Gurobi

julia>
´´´
This corrects my two problem: Gurobi 5.0 not being detected, no error message when it is not found. The test suite was run (include("~/.julia/v0.3/Gurobi/test/runtests.jl")) and ended without error message.