Fix usage of split_inclusive in event parser logic

It was relying on old behavior where Some(empty slice) would be returned
if there were no matches. This was updated in more recent rust versions to return None,
but CircleCI was using a much older version of rust than I have locally.

Updated the usage and CircleCI config to use cimg/rust.

eventsource-client/src/event_parser.rs

@@ -288,16 +288,14 @@ impl EventParser {
         //  * the first line should be appended to the incomplete line, if any
 
         if let Some(incomplete_line) = self.incomplete_line.as_mut() {
-            let line = lines.next().expect("Should not be none!");
-            // split always returns at least one item
-            trace!(
-                "extending line from previous chunk: {:?}+{:?}",
-                logify(incomplete_line),
-                logify(line)
-            );
+            if let Some(line) = lines.next() {
+                trace!(
+                    "extending line from previous chunk: {:?}+{:?}",
+                    logify(incomplete_line),
+                    logify(line)
+                );
 
-            self.last_char_was_cr = false;
-            if !line.is_empty() {
+                self.last_char_was_cr = false;
                 // Checking the last character handles lines where the last character is a
                 // terminator, but also where the entire line is a terminator.
                 match line.last().unwrap() {