Install alpha versions of the ION extension (#680)

There's no stable/beta version published on PECL

MAINTAINERS.md

@@ -61,8 +61,10 @@ We need to monitor new releases at https://jprs.co.jp/idn
 
 ### ion
 
-We manually compile the `ion-c` library.
+- We manually compile the `ion-c` library.
   We need to monitor new releases at https://github.com/amzn/ion-c/releases
+- The only available versions of the `ion` PHP extension are all alpha.
+  We should switch to the stable release once it will be available.
 
 ### ionCube Loader
 

install-php-extensions

@@ -2741,6 +2741,9 @@ installRemoteModule() {
 			fi
 			;;
 		ion)
+			if test -z "$installRemoteModule_version"; then
+				installRemoteModule_version=alpha
+			fi
 			if ! test -f /usr/local/lib/libionc.so || ! test -f /usr/local/include/ionc/ion.h; then
 				echo 'Installing ion-c... '
 				installRemoteModule_src="$(mktemp -p /tmp/src -d)"