Fixed EXP operation

In VM.h the exponential was being calculated incorrectly. This is the simple fix.

Tested using contract:
```
{
	[0x0] "LOOP TESTING"
	(call 0x929b11b8eeea00966e873a241d4b67f7540d1f38 0 0 0 11 0 0)
	(for (MSTORE 0x1 0) (< (MLOAD 0x1) 40) (MSTORE 0x1 (+ (MLOAD 0x1) 1)) (SSTORE (MLOAD 0x1) (* (EXP 2 (MLOAD 0x1)) 0x1)))
}
{
	(call 0x929b11b8eeea00966e873a241d4b67f7540d1f38 0 0 0 0 0 0)
	(suicide)
}
```