Drop support for Go versions older than 1.7 (#172)

* Drop support for Go versions older than 1.7
* Update README.md

.travis.yml

@@ -1,7 +1,7 @@
 language: go
 
 go:
-  - 1.2.x
+  - 1.7.x
   - 1.12.x
   - 1.13.x
   - tip

README.md

@@ -18,6 +18,10 @@ NOTE: can "only" represent numbers with a maximum of 2^31 digits after the decim
 
 Run `go get github.com/shopspring/decimal`
 
+## Requirements 
+
+Decimal library requires Go version `>=1.7`
+
 ## Usage
 
 ```go