Make gasLimit lower than gasLimitString

+ Leaves a margin for cases where tx gas is modified upwards by users (cf: OZ GSN)

lib/api.js

@@ -50,8 +50,8 @@ class API {
 
     this.log = config.log || console.log;
 
-    this.gasLimit = 0xfffffffffff;
+    this.gasLimit = 0xffffffffff;            // default "gas sent" with transactions
-    this.gasLimitString = "0xfffffffffff";
+    this.gasLimitString = "0xfffffffffff";   // block gas limit for ganache (higher than "gas sent")
     this.gasPrice = 0x01;
 
     this.istanbulReporter = config.istanbulReporter || ['html', 'lcov', 'text'];