Update cliques to initialize pivotdonenbrs

pivotdonenbrs is not in a high enough scope to be seen between when it is defined and when it is referenced the second time.  Without this change I get:

```
ERROR: pivotdonenbrs not defined
 in maximal_cliques at /home/user/.julia/v0.3/Graphs/src/cliques.jl:125
while loading solver.jl, in expression starting on line 38
```
Test for undefined-variable bug in maximal_cliques
Replace deprecated IOString with IOBuffer.

IOString was removed from the Julia interpreter in commit ba71cc8.
Merge branch 'maximal_cliques_bug' into a_star