GHC 7.10.1 support

Without it, the following error occurs:

```
src/PgStructure.hs:88:5:
    Non type-variable argument
      in the constraint: Data.String.Conversions.ConvertibleStrings
                           Text k
    (Use FlexibleContexts to permit this)
    When checking that ‘addFK’ has the inferred type
      addFK :: forall k.
               (Ord k, Data.String.Conversions.ConvertibleStrings Text k) =>
               Map.Map k ForeignKey -> Column -> Column
    In an equation for ‘columns’:
        columns table
          = do { cols <- H.listEx
                         $ (\ _1 _2
                              -> Hasql.Backend.Stmt
                                   "select info.table_schema as schema, info.table_name as table_name, info.column_name as name, info.ordinal_position as position, info.is_nullable as nullable, info.data_type as col_type, info.is_updatable as updatable, info.character_maximum_length as max_len, info.numeric_precision as precision, info.column_default as default_value, array_to_string(enum_info.vals, ',') as enum from ( select table_schema, table_name, column_name, ordinal_position, is_nullable, data_type, is_updatable, character_maximum_length, numeric_precision, column_default, udt_name from information_schema.columns where table_schema = ? and table_name = ? ) as info left outer join ( select n.nspname as s, t.typname as n, array_agg(e.enumlabel ORDER BY e.enumsortorder) as vals from pg_type t join pg_enum e on t.oid = e.enumtypid join pg_catalog.pg_namespace n ON n.oid = t.typnamespace group by s, n ) as enum_info on (info.udt_name = enum_info.n) order by position"
                                   (GHC.ST.runST (do { ... }))
                                   True)
                             (qtSchema table) (qtName table);
                 fks <- foreignKeys table;
                 return $ map (addFK fks . columnFromRow) cols }
          where
              addFK fks col = col {colFK = Map.lookup (cs . colName $ col) fks}
```

Also, the regex-tdfa-text library needs a similar patch, but I didn't have time to contact the author yet.

Cheers

src/PgStructure.hs

@@ -1,5 +1,6 @@
 {-# LANGUAGE QuasiQuotes, OverloadedStrings, TypeSynonymInstances,
-             MultiParamTypeClasses, ScopedTypeVariables #-}
+             MultiParamTypeClasses, ScopedTypeVariables,
+             FlexibleContexts #-}
 module PgStructure where
 
 import PgQuery (QualifiedTable(..))