[Fix] nvm_supports_source_options: work around a bug in bash 3.2

See https://github.com/creationix/nvm/issues/1978#issuecomment-453480231

Fixes #1978.

test/fast/Unit tests/nvm_supports_source_options

@@ -0,0 +1,27 @@
+#!/bin/sh
+
+die () { echo "$@" ; exit 1; }
+
+\. ../../../nvm.sh
+
+x=0
+i=0
+runs=5
+calls=500
+first_result=false
+if (nvm_supports_source_options && printf .); then
+  first_result=true
+fi
+while [ $x -le $runs ]; do
+  y=$x
+  x=$(( x + 1 ))
+  while [ $i -le $calls ]; do
+    this_result=false
+    if (nvm_supports_source_options && printf .); then
+      this_result=true
+    fi
+    [ $this_result = $first_result ] || die "call ${i}, run ${y} got ${this_result}; expected ${first_result}"
+    i=$(( i + 1 ))
+  done
+done
+echo "${runs} runs of ${calls} calls succeeded"