Ignore any primary key that is not an integer

since non-integer keys cannot be embedded in the
MVT in the 'id' field. Closes CrunchyData#67

layer_table.go

@@ -451,7 +451,7 @@ func GetTableLayers() ([]LayerTable, error) {
  a.attname AS geometry_column,
  postgis_typmod_srid(a.atttypmod) AS srid,
  trim(trailing 'ZM' from postgis_typmod_type(a.atttypmod)) AS geometry_type,
- coalesce(ia.attname, '') AS id_column,
+ coalesce(case when it.typname is not null then ia.attname else null end, '') AS id_column,
  (
  SELECT array_agg(ARRAY[sa.attname, st.typname, coalesce(da.description,''), sa.attnum::text]::text[] ORDER BY sa.attnum)
  FROM pg_attribute sa