Drop support for Nim 1.4.x

Only support stable Nim versions 1.2.x and 1.6.x.

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        nim: [1.2.16, 1.4.8, stable]
+        nim: [1.2.16, stable]
     steps:
     - name: Checkout
       uses: actions/checkout@v2

testmodule/testContracts.nim

@@ -74,9 +74,7 @@ suite "Contracts":
               holder: Address,
               amount: UInt256): ?TransactionResponse {.contract.}
     let txResp = await token.mint(accounts[1], 100.u256)
-    # `of` instead of `is` due to v1.4.8 compiler error:
-    # "invalid type: 'type Confirmable' for var"
-    check txResp of (?TransactionResponse)
+    check txResp is (?TransactionResponse)
     check txResp.isSome
 
   test "can call non-constant functions with a Confirmable return type":