install: be sure to run cabal update if using cabal

[ci skip]

hledger-install/hledger-install.sh

@@ -48,7 +48,7 @@ HERE
 HLEDGER_INSTALL_TOOL=hledger-install.sh
   # ^ this script's name (can't use $0 when it's piped into bash)
 
-HLEDGER_INSTALL_VERSION=20180217
+HLEDGER_INSTALL_VERSION=20180218
 
 RESOLVER=  #"--resolver=nightly"
   # ^ You can specify a different stackage snapshot here, 
@@ -900,6 +900,9 @@ if [[ $STATUSFLAG ]] ; then
   exit 0
 fi
 
+# if we'll be using cabal, run cabal update once at the start
+(! has_cmd stack && has_cmd cabal && try_info cabal update )
+
 # try installing each package that needs installing, in turn
 echo ----------
 if [[ $(cmd_version hledger) < $HLEDGER_VERSION ]]; then