give operators - and + same precedence

This is a fix for ucsd-progsys/liquidhaskell#155.

`+` used to have higher precedence than `-`, so `(x - y + 1)` was parsed as `(x - (y + 1))`, 
BUT, in haskell `+` and `-` have the same precedence, so the haskell expressions `(x - y + 1) :: {v: a | v = ((x - y) + 1)}`
causing the below constraint to fail 
```
tmp1 = x - y, tmp2 = 1 |- tmp1 + tmp2 => x - (y + 1)
```



"All operators in one list have the same precedence" 
according to http://hackage.haskell.org/package/parsec-3.0.0/docs/Text-Parsec-Expr.html