use env bash rather than /bin/bash

My bash is not installed in /bin/bash.
diff --git "a/test/fast/Running \"nvm use x\" should create and change the \"current\" symlink" "b/test/fast/Running \"nvm use x\" should create and change the \"current\" symlink"
index d5fda3c..fcc4acb 100755
--- "a/test/fast/Running \"nvm use x\" should create and change the \"current\" symlink"
+++ "b/test/fast/Running \"nvm use x\" should create and change the \"current\" symlink"
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 export NVM_SYMLINK_CURRENT=true
 . ../../nvm.sh
diff --git "a/test/fast/Running \"nvm use x\" should not create the \"current\" symlink if $NVM_SYMLINK_CURRENT is false" "b/test/fast/Running \"nvm use x\" should not create the \"current\" symlink if $NVM_SYMLINK_CURRENT is false"
index 4ac8b4b..3603e04 100755
--- "a/test/fast/Running \"nvm use x\" should not create the \"current\" symlink if $NVM_SYMLINK_CURRENT is false"
+++ "b/test/fast/Running \"nvm use x\" should not create the \"current\" symlink if $NVM_SYMLINK_CURRENT is false"
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 . ../../nvm.sh