Minor fix to installation error

[jpdeleon]

I am using Ubuntu: $ uname -a Linux dl 4.13.0-36-generic #40~16.04.1-Ubuntu SMP Fri Feb 16 23:25:58 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

When I tried to install planetplanet: $pip install -U --no-deps planetplanet

I get the following error: #include <gsl/gsl_poly.h> ^ compilation terminated. error: command 'gcc' failed with exit status 1

I have gcc already installed: $ gcc --version gcc (GCC) 4.8.5

So I fixed this issue by installing the GSL library: $ apt-get install gsl-bin libgsl0-dev

I am not sure if this is useful but others might find the same issue in case.