BUG: raise exception if glpk is _not_ installed

The change agrees with the initial version of those lines,
introduced in commit:
    125d8da
This error was introduced in commit:
    caf8c61

polytope/esp.py

@@ -106,7 +106,7 @@ def esp(CC, DD, bb, centered=False, abs_tol=1e-10, verbose=0):
     P = {[C D]x <= b} where C is M x D and D is M x K is
     defined as proj(P) = {x in R^d | exist y in R^k s.t Cx + Dy < b}
     """
-    if 'glpk' in solvers.installed_solvers:
+    if 'glpk' not in solvers.installed_solvers:
         raise Exception(
             "projection_esp error:"
             " Equality set projection requires `cvxopt.glpk` to run.")