Skip pre-release versions when looking for the lastest

Fixes #57.

rust2rpm/__main__.py

@@ -150,14 +150,27 @@ def local_crate(crate, version):
     cratename, version = os.path.basename(crate)[:-6].rsplit("-", 1)
     return crate, cratename, version
 
-def download(crate, version):
-    if version is None:
-        # Now we need to get latest version
+def query_newest_version(crate):
     url = requests.compat.urljoin(API_URL, f"crates/{crate}/versions")
     req = requests.get(url, headers={"User-Agent": "rust2rpm"})
     req.raise_for_status()
     versions = req.json()["versions"]
-        version = next(version["num"] for version in versions if not version["yanked"])
+    for struct in versions:
+        version = struct["num"]
+        if struct["yanked"]:
+            print(f'Ignoring yanked version {version}')
+        elif re.search('alpha|beta|rc|pre', version):
+            print(f'Ignoring pre-release version {version}')
+        else:
+            print(f'Found version {version}')
+            return version
+
+    raise ValueError("Couldn't find any release versions. Specify a version explicitly.")
+
+def download(crate, version):
+    if version is None:
+        # Now we need to get latest version
+        version = query_newest_version(crate)
 
     os.makedirs(CACHEDIR, exist_ok=True)
     cratef_base = f"{crate}-{version}.crate"