gcc-4.9 fix

Sadly libstdc++ before v5 doesn't support moving a stringstream, so work
around it by passing a unique_ptr instead.

eris/serialize/Serialization.cpp

@@ -30,11 +30,11 @@ void Serialization::memory() {
     writeHeader();
 }
 
-void Serialization::memory(std::stringstream &&str, Mode mode) {
-
+void Serialization::memory(std::unique_ptr<std::stringstream> &&str, Mode mode) {
     close();
+    f_ = std::move(str);
+    if (!f_) throw std::runtime_error("Serialization::memory(...) called with a null unique ptr");
 
-    f_.reset(new std::stringstream(std::move(str)));
     f_->exceptions(f_->failbit | f_->badbit);
     if (mode == Mode::OVERWRITE) throw std::logic_error("Serialization::memory(ss, mode) called with invalid mode: Mode::OVERWRITE");
     read_only_ = mode == Mode::READONLY;

eris/serialize/Serialization.hpp

@@ -120,12 +120,16 @@ class Serialization {
      *
      * If an existing file (or memory buffer) is open, it will be closed (or discarded).
      *
-     * \param s the existing stringstream to use for storage.  If the stream contains existing
-     * XZ-compressed data, that data will be decompressed into a new stringstream.
+     * \param s a unique_ptr to the existing stringstream to use for storage.  The Serialization
+     * object takes over management of the pointer (i.e. the caller should not keep it).  If the
+     * stream contains existing XZ-compressed data, that data will be decompressed into a new
+     * stringstream.
      * \param mode the mode of the resulting buffer.  Mode::OVERWRITE may not be used.  Defaults to
      * Mode::READONLY.
      */
-    void memory(std::stringstream &&s, Mode mode = Mode::READONLY);
+    void memory(std::unique_ptr<std::stringstream> &&s, Mode mode = Mode::READONLY);
+    // NB: the above would be much nicer if it just took a std::stringstream&&, but stringstreams
+    // are not move constructible in gcc (actually libstdc++) before 5.0.
 
     /** Returns true if close() may take time due to required copying and/or compression.  Returns
      * false if results have been written directly to the final (uncompressed) file, or if no