Allow both 0.5 and 0.6

Author: jishnub

Project.toml

@@ -21,7 +21,7 @@ Statistics = "10745b16-79ce-11e8-11f9-7d13ad32a3b2"
 
 [compat]
 AbstractFFTs = "0.5, 1"
-ApproxFunBase = "0.6"
+ApproxFunBase = "0.5, 0.6"
 BandedMatrices = "0.16, 0.17"
 BlockArrays = "0.14, 0.15, 0.16"
 BlockBandedMatrices = "0.10, 0.11"